Common electronic devices are constantly in danger of being exposed to damaging voltage transients. These voltage transients may be present in the electric mains due to starting or stopping of large electric motors or to lightening strikes near power lines or the like. Even static electric discharges following, for example, the shuffling of one's shoes across a carpeted surface, may cause damage to integrated circuits such as those found in personal computers.
Traditionally, fuses have been the mainstay for protecting electrical circuits from damage. A fuse might be considered an archetype of a material showing altered conductivity or nonlinear resistance. At normal power loads a fuse exhibits uniforms high conductivity (low resistance), but at higher power loads, the fuse melts opening the circuit. That is, the fuse's conductivity alters (i.e., shows nonlinear resistance) and permanently changes to a high resistance mode. However, dangerous voltage transients can wreak damage in a few nanoseconds--much too short a time for any ordinary fuse to respond. Furthermore, fuses must be replaced before an electronic device can be used again; fuse-like materials that automatically reset themselves are to be much desired.
Fortunately, materials that show extremely rapid alterations in conductivity have been found and can be used to protect electronic circuitry. Materials that show altered conductivity under various conditions are well known in the art of solid state electronics. These materials are mostly semiconductors, since alteration of conductivity forms the basis for virtually all semiconductor electronics. One of the most widely-used materials that shows altered conductivity is a metal oxide "varistor" (MOV). This semiconductor device exhibits extremely high resistance when exposed to relatively low voltages like those of domestic electric supplies (i.e. 120 V). However, this same device rapidly switches to a good conductor when exposed to higher voltages. MOV materials can be custom made allowing a fairly precise choice of the potential "clamp" voltage (the voltage at which the device switches into a conducting mode).
These materials are usually employed as potential parallel pathways between a voltage supply and ground. Under normal conditions the MOV behaves as an insulator, but when a voltage transient exceeds the clamp voltage, the MOV becomes a good conductor and the transient is harmlessly conducted through the MOV to ground. The MOV then rapidly reverts to its nonconducting state until the next voltage transient arrives.
One problem with MOV materials is that they tend to have relatively high capacitance, which results in a longer than optimal switch-over time. This and related problems have been addressed through the production of altered conductivity materials produced from conducting and semiconducting materials suspended in an insulating matrix such as an epoxy plastic. These materials are reported to have superior properties over traditional MOV devices.
U.S. Pat. No. 4,726,991 to Hyatt et al. discloses a mixture of conductive particles (carbonyl nickel, nickel, tantalum carbide, gold, silver, etc.) in the size range of 100 .mu.m coated with an insulator such as silicon dioxide and mixed with similarly-sized semiconductor particles (doped silicon, selenium, germanium, etc.) and suspended in an insulating binder such as organic polymers like epoxy. This material is reported to have a more rapid response time than MOV devices and can be made with clamping voltages as low as 5 V.
U.S. Pat. No. 4,992,333 to Hyatt discloses another system based on a packed mixture of conducting, semiconducting and insulating particles, all embedded in a nonconducting matrix material. The materials are similar in nature to the earlier Hyatt et al. patent. In this case, however, the conducting particles are in the 100 .mu.m range, the semiconducting particles are in the micrometer range, and the insulating particles are in the submicrometer range. The patent also discloses dimensions and packing ratios necessary to achieve particular results. A similar nonlinear resistance material is disclosed in U.S. Pat. No. 4,977,357 to Shrier, which teaches a material formed from conducting particles uniformly dispersed in an insulating binder.
All of the above-mentioned nonlinear resistors show low conductivity at low potentials and high conductivity at higher potentials. Generally, it is believed that these and similar materials operate by means of quantum electron tunneling. That is, they are all arranged with the conducting particles slightly separated so that the materials exhibit high electrical resistance. As higher potentials are imposed across the materials, electrons "tunnel" through the insulator and "jump" from conducting particle to conducting particle, thus causing the material to switch into the high conductance or low resistance mode. As soon as the potential falls, the electrons are no longer sufficiently energetic to "tunnel" so that the material regains its original high resistance.
The drawback with all of the nonlinear resistors discussed heretofore is that they all transition from high resistance at low potential to low resistance at high potential. While this behavior may be ideal for shunting a power supply to ground, it cannot solve all voltage transient problems.
There are also available a number of "repetitive fuses" (such as PolyFuse manufactured by Raychem Corporation of Menlo Park, Calif.); these devices are variable resistors whose resistance greatly increases with increases of temperature. If a circuit draws excess current through one of these devices, the device will increase in temperature. This causes the resistance to increase greatly, thereby cutting off the circuit from the power source. However, such devices are slow acting and unable to provide protection from brief, but damaging, voltage transients.
High impedance signal inputs such as the gates of field effect transistors (FET) and other similar devices remain sensitive to damage by excessive voltages. These devices can be readily damaged by high voltage pulses or even static electric discharges. Even when such inputs are connected to ground by one of the nonlinear resistors already discussed, damage may occur before the nonlinear resistor can change state and shunt the voltage pulse to ground. Further, many hand-held instruments may lack a clear path to ground rendering these protective devices ineffectual. Furthermore, the need for ground shunts may result in excess fabrication cost or may even alter the operation of some circuits. Having the input shunted to ground by each voltage transient may also tend to introduce noise into the circuit. What is needed is a nonlinear resistor that changes state in the opposite direction; that is, one that conducts at low potentials and rapidly develops high resistance at higher potentials. Such a resistor could be placed in series with the input and effectively cut off the input whenever the signal contained excessively high voltage.